1- : the main difference is that ...e5 gives more support to a kingside attack, but ...c5 does not block the g7 bishop. For instance, in the Saemisch variation (White plays f3 and follows by a kind of Yugoslav attack - cf. the Dragon Sicilian) ...e5 is often a mistake because Black has to create problems on the queenside if White castles here.

2- That order of moves is a bit odd. But anyways, White refrains from playing d5 as long as he can because it would block the center ; as long as the center is not blocked Black cannot attack on the kingside by ...f5. Additionnaly, the knight is better on d7 than e7 because it keeps an eye on the c5 square. Sometimes even (7...Nc6 8.d5) 8...Nb8 is played, precisely to play that knight on e7.

EDIT : of course I forgot to mention that ...Nc6, attacking d4, more or less forces d5.

If you still struggle with advanced opening concepts, not specific lines, I recommend Ludek Pachman's book of openings : don't trust the deep lines since theory has changed much since then, but you can learn a lot from it (weak squares, time/material/space trade-off, etc.).

If you are more advanced and seek specific material for this opening, I can't help you. What I've learnt about it comes mostly from my practice.

You can generally use either ...e5 or ....c5. Playing ...c5 often leads to a Ben-oni or Benko gambit type opening. Most KID players play for ...e5 and an attack on the King side. If White clearly commits to the Queenside then ...e5 becomes a race variation that Black doesn't have to be first in order to the win the game. If White instead of committing to the Queenside plays a blockading system (for instance the Saemisch Attack) or a broad center (for instance the Four Pawn Attack) then ...c5 starts becoming attractive. When White doesn't play c4 but, plays something like the Colle, Trompowsky, Torre, Veresov, or London where he is looking to attack Kingside with a prepared e4, then ...c5 also should be considered. It is good to know about pawn structures and thematic breaks associated with pawn structures before going deeply into opening theory.